An elaborate pathway required for Ras-mediated epigenetic silencing

Abstract

The conversion of a normal cell to a cancer cell occurs in several steps and typically involves the activation of oncogenes and the inactivation of tumour suppressor and pro-apoptotic genes. In many instances, inactivation of genes critical for cancer development occurs by epigenetic silencing, often involving hypermethylation of CpG-rich promoter regions. It remains to be determined whether silencing occurs by random acquisition of epigenetic marks that confer a selective growth advantage or through a specific pathway initiated by an oncogene. Here we perform a genome-wide RNA interference (RNAi) screen in K-ras-transformed NIH 3T3 cells and identify 28 genes required for Ras-mediated epigenetic silencing of the pro-apoptotic Fas gene. At least nine of these RESEs (Ras epigenetic silencing effectors), including the DNA methyltransferase DNMT1, are directly associated with specific regions of the Fas promoter in K-ras-transformed NIH 3T3 cells but not in untransformed NIH 3T3 cells. RNAi-mediated knockdown of any of the 28 RESEs results in failure to recruit DNMT1 to the Fas promoter, loss of Fas promoter hypermethylation, and derepression of Fas expression. Analysis of five other epigenetically repressed genes indicates that Ras directs the silencing of multiple unrelated genes through a largely common pathway. Last, we show that nine RESEs are required for anchorage-independent growth and tumorigenicity of K-ras-transformed NIH 3T3 cells; these nine genes have not previously been implicated in transformation by Ras. Our results show that Ras-mediated epigenetic silencing occurs through a specific, complex, pathway involving components that are required for maintenance of a fully transformed phenotype.

Fas expression at diagnosis as a biomarker of azacitidine activity in high-risk MDS and secondary AML

“…Responses were scored according to the International Working Group (IWG) 2006 criteria for MDS. 10 Patients having stable disease with hematological improvement (HI) were considered as responders, and those without HI were considered as non-responders. Flow cytometry analysis of Fas was performed in 38 patients (13 sAML and 25 MDS) at diagnosis, just before treatment initiation and after X3 cycles of treatment (Supplementary Table S4 and Figure 1d).…”

Régulation de la survie cellulaire par les kinases de la famille RAF

“…Dans ce dernier cas, l'inhibition en trans de ROK par C-RAF semble capable d'influencer la structure du cytosquelette d'actine et, indirectement, la signalisation par les récepteurs de mort cellulaire comme FAS [24]. [28]. Il sera important de déterminer dans quelle mesure les kinases RAF régulent la structure et l'accessibilité de la chromatine.…”

Profils de méthylation de l’ADN dans les cellules normales et cancéreuses

“…Une étude récente favorise plutôt la seconde hypothèse. Cette étude suggère que l'oncogène Ras induit directement la méthylation de gènes cibles via le recrutement de plusieurs effecteurs épigénétiques [16]. Une autre piste de recherche très prometteuse est la possible influence de l'environnement sur les profils de méthylation.…”